A catalytic hydrocarbon fuel reformer converts oxygen and a fuel comprising, for example, natural gas, light distillates, methanol, propane, naphtha, kerosene, gasoline, diesel fuel, bio-diesel or combinations thereof, into a hydrogen-rich reformate stream comprising a gaseous blend of hydrogen, carbon monoxide, and nitrogen, plus trace components. In a typical reforming process, the hydrocarbon fuel is percolated with oxygen in the form of air through a catalyst bed or beds contained within one or more reactor tubes mounted in a reformer vessel. The catalytic conversion process is typically carried out at elevated catalyst temperatures in the range of about 700° C. to about 1100° C.
The produced hydrogen-rich reformate stream may be used, for example, as the fuel gas stream feeding the anode of an electrochemical fuel cell. Reformate is particularly well suited to fueling a solid-oxide fuel cell (SOFC) system because a purification step for removal of carbon monoxide is not required as in the case for a known proton exchange membrane (PEM) fuel cell systems.
The reformate stream may also be used in spark-ignited (SI) or diesel engines. Reformate can be a desirable fuel or fuel-additive; the reformate stream also can be injected into the vehicle exhaust to provide benefits in reducing vehicle emissions. Hydrogen-fueled vehicles are of interest as low-emissions vehicles because hydrogen as a fuel or a fuel additive can significantly reduce air pollution and can be produced from a variety of fuels. Hydrogen permits a SI engine to run with very lean fuel-air mixtures that greatly reduce production of NOx. As a gasoline additive, small amounts of supplemental hydrogen-rich reformate may allow conventional gasoline-fueled internal combustion engines to reach nearly zero emissions levels. As a diesel fuel additive, supplemental reformate may enhance operation of premixed combustion in diesel engines. Reformate can be injected into the vehicle exhaust stream to improve NOx reduction and/or as a source of clean chemical energy for improved thermal management of exhaust components (for example, NOx traps, particulate filters and catalytic converters).
Fuel/air mixture preparation constitutes a key factor in the reforming quality of catalytic reformers, and also the performance of porous media combustors. A problem in the prior art has been how to vaporize fuel completely and uniformly, especially at start-up when the apparatus is cold. A related problem is that injected fuel droplets may follow a line-of-sight path directly to the entry surface of the catalyst, resulting in extreme, localized fuel/air inhomogeneities. Inhomogeneous fuel/air mixtures can lead to decreased reforming efficiency and reduced catalyst durability through coke or soot formation on the catalyst and thermal degradation from local hot spots. Poor fuel vaporization can lead to fuel puddling, resulting in uncertainty in the stoichiometry of fuel mixture. Complete and rapid fuel vaporization well ahead of the catalyst is a key step to achieving a homogeneous gaseous fuel-air mixture and consequent efficient reformate generation.
Fuel vaporization is especially challenging under cold start and warm-up conditions for a fuel reformer. In the prior art, it is known to vaporize injected fuel by preheating the incoming air stream to be mixed with the fuel, or by preheating a reformer surface for receiving a fuel spray. However, none of the prior art approaches is entirely successful in providing reliable, complete vaporization of injected liquid fuel under start-up conditions.
During start-up in a typical prior art fast light-off reformer, fuel and air are mixed stoichiometrically (or nearly-stoichiometrically) and burned in the fuel/air mixing chamber, and the hot combustion products are passed through the catalyst bed. This combustion phase provides the initial energy required to light-off the reforming catalyst and heats the fuel/air mixing zone to assist in fuel vaporization.
After a brief combustion period, typically about 2 to 20 seconds, combustion is quenched and a very rich fuel/air mixture is supplied to initiate reformate production. The atomized fuel mixes with the airflow within the volume defining the mixing zone prior to reacting within the catalyst. The energy generated during the reforming process (exothermic reaction) continues to heat the reformer, including a heat exchange section downstream of the reforming catalyst. Under warmed-up operation, the heat exchange section transfers heat from the hot reformate gas to the incoming airflow. This heat exchange provides energy to the mixing zone to assist fuel vaporization.
After the end of combustion but while the reformer is warming up, a transitional heat deficit develops in heat energy available in the mixing chamber for fuel vaporization. This deficit arises because the heat energy stored in the mixing section of the reformer during the combustion stage is depleted during early reforming before the heat exchange section is sufficiently warm to provide substantial heat from the reforming process back into the incoming airflow. The extent and duration of this deficit is dependent upon a number of factors, including heat generated and stored during combustion, the thermal mass of the catalyst and heat exchange section, and heat transfer rates within the reformer assembly. The maximum temperature that the catalyst face can sustain without thermal degradation of the catalyst, which typically is about 1100-1200° C., limits the duration of combustion, which thus limits the amount of energy that may be stored and available for fuel vaporization during early reforming.
What is needed in the art is a compact reformer arrangement that provides sufficient volume, residence time, and heat to accomplish good fuel/air mixing and heating following a combustion phase during warm up of a hydrocarbon catalytic reformer.
It is a primary object of the invention to reduce or eliminate the transitional heat deficit experienced by prior art reformers during start-up of the reformer.